theorem Th23:
  Seg 10 = {1,2,3,4,5,6,7,8,9,10}
  proof
    thus Seg 10 = Seg 9 \/ {9+1} by FINSEQ_1:9
    .= {1,2,3,4,5,6,7,8,9,10} by Th22,ENUMSET1:85;
  end;
